The efficacy of a number of adenine nucleosides which act as both antitumor and antiviral agents is severely limited due to their rapid deactivation in vivo by the action of adenosine deaminase, an enzyme present in most mammalian body tissues. The compounds (R)-3-(2-deoxy-beta-D-erythropentofuranosyl)-3,6,7,8-tetrahydroimidazo[4,5 -d][1,3]diazepin-8-ol (commonly known as pentostatin), disclosed in U.S. Pat. No. 3,923,785, and its ribo-analog, (R) -3-(beta-D-ribofuranosyl)-3,6,7,8-tetrahydroimidazo[4,5-d][1,3]diazepin-8- ol (commonly known as coformycin), disclosed in Japanese Pat. No. 875,639 and U.S. Pat. No. 4,151,347, are potent inhibitors of adenosine deaminase.